The rotor shaft of an electrical machine is subject to bending during rotation. The critical speed of the rotor shaft (sometimes called its natural frequency) is the rotational speed at which the rotor shaft becomes dynamically unstable and where it is likely to experience significant structural resonances which may cause noise, vibration, fatigue stresses and other undesirable effects. For any particular rotor shaft there are a large number of critical speeds but often only the first (lowest) and sometimes the second (next lowest) will be of concern. The remaining critical speeds will normally be so high as to be outside the operating speed range of the electrical machine.
Electrical machines may have critical speeds that limit their operating speed range. This can be a particular problem for variable speed electrical machines and especially those that have a high operating speed where to reach that operating speed a critical speed must be passed. The critical speed is typically proportional to the stiffness and mass distribution of the rotor shaft and supporting bearing. It can be difficult to avoid critical speeds because there are often constraints on the design of the electrical machine such as the type of supporting bearing that must be used, the position of the supporting bearing, and the rotor shaft materials.
Existing solutions to the problem of critical speed are directed to providing an improved dynamic response for the supporting bearings and structures.